1. Field of the Invention
The invention relates to an aluminum nitride ceramics and a member for use in the production of semiconductors.
2. Related Art Statement
An electrostatic chuck system utilizing the Johnson-Rahbek effect is useful for the adsorption and retention of semiconductor wafers. In such a system, the volume resistivity of the substrate of the chuck is preferably 108 to 1013 Ω·cm for improved adsorption force and speed of response. It is therefore desirable to control the volume resistivity of the substrate within 108 to 1013 Ω·cm in the temperature range intended for use, in the development of an electrostatic chuck.
The applicant filed Japanese patent publication No. 9-315867A and disclosed that the volume resistivity of aluminum nitride with a high purity may be adjusted to 108 to 1013 Ω·cm at room temperature by adding a small amount of yttrium oxide thereto. The effects of the addition of a rare earth elements other than yttrium into an aluminum nitride sintered body on its volume resistivity was not studied in the Japanese patent publication No. 9-315867A.
Japanese patent publication No. 63-46032B discloses a process for producing an aluminum nitride sintered body. According to the claim 1, aluminum nitride powder containing 1 weight percent of oxygen is mixed with 0.01 to 15 weight percent of the oxide of a metal element selected among yttrium, lanthanum, praseodymium, niobium, samarium, gadolinium and dysprosium to obtain mixed powder. The powder is then shaped and sintered to obtain an aluminum nitride sintered body having a high thermal conductivity and containing 0.01 to 20 weight percent of oxygen. According to “example 1” in the publication, aluminum nitride powder (with a mean particle diameter of 1 μm) containing 1 weight percent of oxygen is mixed with 3 weight percent of samarium oxide to obtain mixed powder. The powder is then subjected to hot press at a pressure of 300 kg/cm2 and a temperature of 1800° C. for 1 hour to obtain a sintered body with a thermal conductivity of 121 W·m/k at room temperature.
In Japanese patent publication 63-46032A, a rare earth element is added to raw powder of aluminum nitride for improving the thermal conductivity of the resultant aluminum nitride sintered body, but the effect of the addition on the volume resistivity of the sintered body was not studied. Further, in Japanese patent publication 63-46032A, samarium oxide is added to raw material of aluminum nitride in a content of 3 weight percent calculated as samarium to obtain mixture, which is then hot pressed to obtain an aluminum nitride sintered body, but the change of the volume resistivity of the sintered body due to the addition of samarium oxide was not studied.
Further, the applicant disclosed, in a U. S. Patent publication 2002-0110709A1 (now U.S. Pat. No. 6,607,836), an aluminum nitride sintered body having a content of samarium of 0.04 mol percent or more so that the body has a samarium-aluminum oxide phase. The oxide phase is interconnected to result in a volume resistivity at room temperature of 1012 Ω·cm or lower. According to the description in U. S. Patent publication 2002-0110709A1 (now U.S. Pat. No. 6,607,836), however, it is necessary that the body contains 0.04 mole percent or more of samarium calculated as samarium oxide, for attaining a volume resistivity at room temperature of about 1012 Ω cm. As the content of samarium is larger, the volume resistivity at room temperature is lower. U. S. Patent publication 2002-0110709A, however, does not disclose any microstructure having a samarium-aluminum composite oxide phase of a length of 7 μm or longer formed with a such a small content of samarium.
However, in an application for producing semiconductors such as a substrate for an electrostatic chuck, it is indispensable to eliminate the contamination of the semiconductors by metal elements. It is thus desirable to reduce the content of a heavy metal other than aluminum in the aluminum nitride sintered body as possible.